This work analyses the legal challenges posed by contemporary practices of extraterritorial immigration control: visas, pre-embarkation checks and the interception of irregular migrants. It examines the international law framework, and provides case-studies from Europe, Australia and the United States. A central element of contemporary border regimes is their application to migrants before they reach a state's territory. The main forms of this extraterritorial immigration control are visa requirements, pre-embarkation immigration controls and the interception of irregular migrants at sea. This work analyses the complex relationship of the law to these practices, as legal guarantees are potentially avoided, while the legality of control is often uncertain. It examines the international law framework, including the law of the sea and the extraterritorial application of principles of non-refoulement contained in the Refugee Convention and in international human rights law. The work also includes detailed case-studies of the legal challenges posed by extraterritorial immigration controls in Europe, Australia and the United States.